I would like to modify Android OS (official image from AOSP) to add preprocessing to a normal phone call playback sound.

I&#39;ve already achieved this filtering for app audio playback (by modifying *HAL* and *audioflinger*).

I&#39;m OK with targeting only a specific device (Nexus 5X). Also, I only need to filter playback - I don&#39;t care about recording (uplink).

*UPDATE #1:* 

To make it clear - I&#39;m OK with modifying Qualcomm-specific drivers, or whatever part that it is that runs on Nexus 5X and can help me modify in-call playback.


*UPDATE #2:* 

I&#39;m attempting to create a Java layer app that routes the phone playback to the music stream in real time.

I&#39;ve already succeeded in installing it as a system app, getting permissions for initializing `AudioRecord` with `AudioSource.VOICE_DOWNLINK`. However, the recording gives blank samples; it doesn&#39;t record the voice call.

This is the code inside my worker thread:

    // Start recording
    int recBufferSize = AudioRecord.getMinBufferSize(44100, AudioFormat.CHANNEL_IN_STEREO, AudioFormat.ENCODING_PCM_16BIT);
    mRecord = new AudioRecord(MediaRecorder.AudioSource.VOICE_DOWNLINK, 44100, AudioFormat.CHANNEL_IN_STEREO, AudioFormat.ENCODING_PCM_16BIT, recBufferSize);

    // Start playback
    int playBufferSize = AudioTrack.getMinBufferSize(44100, AudioFormat.CHANNEL_OUT_STEREO, AudioFormat.ENCODING_PCM_16BIT);
    mTrack = new AudioTrack(AudioManager.STREAM_MUSIC, 44100, AudioFormat.CHANNEL_OUT_STEREO, AudioFormat.ENCODING_PCM_16BIT, playBufferSize, AudioTrack.MODE_STREAM);

    mRecord.startRecording();;
    mTrack.play();

    int bufSize = 1024;
    short[] buffer = new short[bufSize];
    int res;
    while (!interrupted())
    {
        // Pull recording buffers and play back
        res = mRecord.read(buffer, 0, bufSize, AudioRecord.READ_NON_BLOCKING);
        mTrack.write(buffer, 0, res, AudioTrack.WRITE_BLOCKING);
    }

    // Stop recording
    mRecord.stop();
    mRecord.release();
    mRecord = null;

    // Stop playback
    mTrack.stop();
    mTrack.release();;
    mTrack = null;


I&#39;m running on a Nexus 5X, my own AOSP custom ROM, Android 7.1.1. I need to find the place which will allow call recording to work - probably somewhere in `hardware/qcom/audio/hal` in platform code.

Also I&#39;ve been looking at the function `voice_check_and_set_incall_rec_usecase` at `hardware/qcom/audio/hal/voice.c` However, I wasn&#39;t able to make sense of it (how to make it work the way I want it to).

*UPDATE #3:*

  I&#39;ve opened [a more-specific question](https://stackoverflow.com/questions/43818736/call-recording-make-audiosource-voice-downlink-work-on-nexus-5x-rooting-or-cu) about using `AudioSource.VOICE_DOWNLINK`, which might draw the right attention and will eventually help me solve this question&#39;s problem as well.


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Modifying in-call voice playback in Android custom ROM